GENEVA — The United Nations mediator in the war in Syria met Monday in Geneva with members of the opposition, as deaths from a suicide attack on a Shi’ite shrine in a government-controlled territory escalated sharply.

Opposition members said the meeting with the mediator, Staffan de Mistura, lasted nearly two hours and they had received a “positive response’’ from him to their demands in creating the basis for talks with the Syrian government: a release of political prisoners, the lifting of sieges on insurgent-held areas, and a halt to airstrikes.

As a first step, they presented de Mistura with a list of 300 people, all women and children, whom they described as political prisoners of the forces loyal to President Bashar Assad.

The suicide attack Sunday on the revered shrine, Sayeda Zeinab, near the Syrian capital, Damascus, killed at least 72 people, according to the Syrian Observatory for Human Rights, a monitoring group. About 50 deaths had been reported earlier.

The Islamic State militant group claimed responsibility for the attack, which heightened the potential to sharpen sectarian divides in the conflict. The Islamic State, which controls parts of Syria and Iraq and is widely reviled as a Sunni extremist terrorist organization, is not a party to the Geneva talks, nor does it have any interest in a political settlement.

In Moadhamiyeh, a rebel-held suburb of Damascus hit by heavy bombardment on Sunday, doctors said investigations had shown that breathing difficulties in patients had been caused by dust and phosphates and not from a chemical weapon like chlorine, as they had initially feared.

The Syrian government intensified its military advance even as it took steps to talk in Geneva. On Monday, a shell hit a school playground in the rebel-occupied town of Madaya, wounding several children, according to residents.

The government has for months laid siege to Madaya, where some residents, including children, have starved because the delivery of emergency food and medicine has been obstructed.

The United Nations has said the government is responsible for besieging roughly 187,000 people in rebel-held towns, while opposition fighters are besieging two towns with around 12,000 residents.

Until now, a cease-fire had held in Madaya and neighboring Zabadani, along with two government-held towns to the north that are surrounded by insurgents. The renewed military strikes on Madaya began in recent days, along with the talks in Geneva, as the warring parties jockey for leverage at the table.

Umm Majd, a Madaya resident reached by telephone who gave only her nickname out of concerns for her safety, called on Riad Hijab, the head of the opposition High Negotiations Committee, to pull out of the Geneva talks because the cease-fire had been broken.

“What negotiations are they talking about?’’ she said. “We’re living in starvation and blood. Before the conference there was a truce. After it we have shelling.’’

Her anger reflected the pressure on the opposition delegation as it started talks with de Mistura. Opposition fighters have lately lost ground on the battlefield, mainly because of Russian airstrikes in support of government forces.

At the same time, the opposition has been eager to show willingness to start talks to end the nearly five-year war.

The government delegation first talked with de Mistura on Friday, and will see him again.

Brett McGurk, President Obama’s envoy to the US-led coalition fighting the Islamic State, said he visited northern Syria over the weekend to review the ongoing fight against the extremist group.

It was the first visit by a senior administration official to Syria since the beginning of the campaign against ISIS in August 2014.